The present invention relates to an automatic focusing device suitable for use in a photographic or television camera having an objective lens assembly axially movable in predetermined incremental displacements in search of the true focus setting in relation to the subject to be photographed, and a method therefor.
According to a prior art focusing system, employed in a photographic camera, a scanning device is employed for scanning an image of the subject to be photographed (hereinafter referred to as a target subject) that has been projected through the objective lens assembly of the camera onto an image plane which essentially corresponds to, or is an equivalent to, the plane where a light sensitive film is placed. The scanning device referred to above comprises a combination of a photo-transducer and a mechanical interceptor in the form of either a slit or a pin hole formed in a rotatable disc which is located in front of the photo-transducer.
In this prior art focusing system, automatic focusing is carried out in such a way that a serial video signal indicative of the image of the target subject, that has emerged from the photo-transducer, is differentiated to give an indication, depending on the value of the differentiated signal, whether or not the image of the target subject is focused in the image plate. The true focus setting can be achieved by varying the distance between the objective lens assembly and the image plane in such a way as to increase the level of the differentiated signal to an extreme value and, at the time the differentiated signal attains the extreme value, the image of the target subject is considered exactly focused in the image plate.
The employment of a multi-cell photo-transducer, such as a self-scanning photo-diode array or image sensor array, having a plurality of transducer elements, electirically connected either in a linear arrangement or in a matrix arrangement, is possible in place of the combined photo-transducer and interceptor. As is well known to those skilled in the art, the multi-cell photo-transducer has a built-in shift register and self-scans in such a way that the transducer elements are sequentially brought into operation in response to clock pulses applied thereto through the shift register.
The transducer elements of the multi-cell transducer are sequentially brought into operation to scan the image of the target subject in a substantially similar way as the combined transducer and interceptor does and, therefore, each transducer element of the multi-cell transducer may be considered as corresponding to the slit or pin hole of the mechanical interceptor. In view of the foregoing, for the sake of convenience, the term "unitary light receptor (of the scanner)" hereinafter employed should be understood as intended to include any one of the transducer elements of the multi-cell transducer and the slit or pin hole of the interceptor.
In the arrangement of the prior art focusing system, referred to above, the scanner is designed such as to detect a relatively high component of the spatial frequency of the image of the target subject and, therefore, it has often been found that focusing of an image of the target subject which is composed of a relatively low spatial frequency, i.e., of the target subject having a blurring contrast, is hardly performed. Moreover, detection of the relatively high spatial frequency for focusing essentially requires a decrease of the size of a unitary light receptor of the scanner employed, but practice has shown that the extent to which the size of the unitary light receptor can be reduced is limited. In other words, the smaller the size of the unitary light receptor, the lower the output energy from the unitary light receptor and, therefore, the time response characteristic thereof is so lowered that the focusing system can not be practically employed in a photographic camera which is often used in taking pictures of dark target subjects.
In addition, so far as focusing in relation to a target subject having depth and brightness distribution is conerned, the prior art system often fails to determine a definite focus position and sometimes, focusing of a selected area of the target subject in the image plane is settled down in relation to one of the other areas of the target subject which are relatively close to the selected area. Therefore, this also constitutes one of the major reasons for reduction of practical availability of the prior art system.